Supercritical Fluid Chromatography (SFC) and Supercritical Fluid Extraction (SFE) apparatus typically include pumping systems for pumping highly compressible fluids (for example, liquified CO.sub.2), at flow rates on the order of 5 g/min. Maintaining the pumping fluid at sub-ambient temperatures prior to pumping reduces the compressibility of a compressible pumping fluid such that it can be accurately pumped.
It is known that direct application of a cryogenic fluid may be used to cool the pump head of pumping apparatus, however, an independent source of cryogenic fluid is typically employed. U.S. Pat. No. 5,142,875 entitled "Cooled Pumping System" discloses a single source of cryogenic fluid applied to a heat exchanger through cryogenic input nozzle 112 to simultaneously cool the pump head and the pumping fluid prior to and during pumping, thus reducing the bulk modulus of the pumping fluid. Since the compression stage is now isothermal, tile solvent delivery is relatively accurate and the mass flow can be easily calculated using tile first law of thermodynamics and mass conservation equations. In particular, the pump head 110 acts as a heat exchanger in which cryogenic fluid may be supplied to a cryogenic input nozzle 112 and expanded to ambient pressure. The expanded cryogenic fluid simultaneously cools the pre-cooler supply tube 116, the pump head 110 and ultimately, the pumping fluid which is input through the pumping fluid input 118. The pump head 110 is made of a material having a much higher thermal conductivity than the rest of the pump body. A thermal insulator 120 is disposed between the pump head and the pump body such that the pump head is effectively thermally insulated from the pump body. The precooler supply tube 116 is thermally coupled to a channel 117 along the top of the pump head. Since the length of tube is relatively long in relation to the tube diameter, it acts as a heat exchanger and dissipates heat from the pumping fluid to the pump head.
The use of thermoelectric cooling elements to transfer heat from a pump head and to a heat sink is disclosed in the prior art pumping system illustrated in FIG. 2 and is fully described in U.S. Pat. No. 5,180,293 entitled "Thermoelectric Pumping System". The pumping system includes a pump head 220 which is thermally isolated from the body of the pump. Since the temperature of the pumping fluid after pumping does not affect the accuracy with which the fluid is pumped, a counterflow heat exchanger 210 is employed to utilize the relatively cool pumping fluid exiting the pump to precool the fluid entering the pump prior to pumping. A second heat exchanger 230 is coupled to the pump head 220 and thermoelectric elements 240 in very close proximity to the pump head inlet such that the thermoelectric elements 240 reduce the temperature of the pumping fluid just prior to pumping. Thermoelectric elements 240 pull heat from the pump head such that it can be dissipated by the heat sink 250. Thermoelectric elements have the ability to generate a temperature differential of approximately 70 degrees centigrade between their hot and cold sides. However, the total amount of heat they are capable of removing is inversely related to the temperature differential across them. As the ambient temperature surrounding the pumping system approaches 70 degrees centigrade above the temperature of the pumping apparatus, thermoelectric cooling becomes inefficient and accurate temperature regulation of the pumping fluid is difficult.